Merge branch 'ab/mktag-tests'

Fill test gaps.

* ab/mktag-tests:
  mktag tests: test fast-export
  mktag tests: test for-each-ref
  mktag tests: test update-ref and reachable fsck
  mktag tests: test hash-object --literally and unreachable fsck
  mktag tests: invert --no-strict test
  mktag tests: parse out options in helper